Capacitance-responsive probes, wherein a capacitance sensor develops a capacitance which is a measure of liquid level, displacement, moisture content or other similar quantities, are a well-known art. An example is U.S. Pat. No. 3,831,069 to Merrell et al. which discloses a level detecting probe with electrical circuitry enclosed in the probe housing. The Merrell disclosure does not contemplate the use of the circuitry associated with the probe for purposes other than liquid level sensing, nor does Merrell disclose the use of a compensator to handle extraneously-produced capacitance in the measurement and reference signals. The circuitry of Merrell et al. merely provides a signal indicative of threshold liquid level, not a signal indicative of the height of liquid level. A fortiori, Merrell et al. does not teach the use of circuitry to produce an output signal which is substantially linearly proportional to the capacitance or differential capacitance sensed by the probe or probes. Neither do any of the other patents cited and discussed below (except Wallman and DiNiro) produce an output signal varying substantially linearly with "sensed" capacitance.
Another example is U.S. Pat. No. 3,805,150 to Abbe, which teaches the use of two axially displaced probe tips to measure displacement or standoff distance of one probe tip (the nearest) from a grounded conducting surface. Again, the Abbe specification contemplates use of the invention for measurement of a single type of quantity (displacement), although compensation for some sources of extraneous capacitance is included.
U.S. Pat. No. 3,728,897 to Wallman discloses a temperature sensing and density (or mass) sensing gauge for liquid fuel which uses fuel temperature and capacitance of the fuel as measured intermediary quantities. Compensation is provided for variations in fuel temperature as it affects capacitance of the fuel. Although Wallman's output signal varies nearly linearly with capacitance, no compensation for extraneous capacitance is provided.
The DiNiro Patent, U.S. Pat. No. 3,626,287, discloses a probe which measures capacitance change, with compensation provided for the presence of stray capacitances, which is suitable for measuring linear displacement between two or more movable capacitative plates. The DiNiro compensator is a specially wound transformer with low primary to secondary leakage capacitance. DiNiro discusses and explicitly rejects a method closely related to the compensation method of the subject invention, namely a shield driver which drives the potential of each probe shield to match that of the inner conducting wire of the corresponding probe. DiNiro does not use a balancing or reference probe against which measured capacitance is compared.
U.S. Pat. No. 3,300,718 to Umphrey discloses use of a shield driver to reduce distributed capacitance of the probe cables which carry the probe signal, as does U.S. Pat. No. 3,619,742 to Rud.
U.S. Pat. No. 3,582,728 to Thoma discloses a moisture or humidity sensing element, comprising a moisture-sensitive dielectric core which has moisture-permeable electrode layers bonded to each of two opposite surfaces of the core. No provision is made for compensation of extraneously-introduced capacitance, the electrode layers must be water-permeable and the device is suitable only for sensing one type of physical quantity, viz. moisture content. U.S. Pat. No. 3,350,941 to Misevich et al. is similar in approach to Thoma.
Another moisture content sensor, disclosed in U.S. Pat. No. 3,523,255 to Goodman et al., uses an aluminum base and a thin, electrically conducting metallic layer as electrodes and a thin, porous layer of aluminum oxide as the moisture-absorbent dielectric therebetween. The device is evidently useful only for measurement of moisture content, and no extraneous capacitance compensation is included.
Moffatt, in U.S. Pat. No. 3,405,559, discloses a pressure sensing element, whereby differential pressure on one capacitive membrane or thin plate causes the membrane to deform towards or away from a second initially parallel capacitive plate and so change the capacitance of the two plates by a measurable amount. No extraneous capacitance compensation or alternative uses are disclosed.
It is an object of the invention herein described to provide apparatus for sensing "general displacement" (as defined herein), the apparatus comprising pulse generation means, resistive-capacitive circuit means including a probe for producing a signal indicative of "general displacement," second resistive-capacitive circuit means for producing a reference "general displacement" signal, integration means for the two signals, amplification means for each signal, shield driver means for shielding the probe of the first resistive-capacitive circuit means from extraneous capacitance effects, and output means for producing a signal varying as a mathematical function of the "general displacement" sensed by the probe.
Other objects, features and advantages of the invention will become apparent from reference to the following drawings and detailed description .